BARCELONA (Reuters) - T-Mobile US will propose a "significant" share buyback that could start in December, CFO Braxton Carter said on Thursday, a sign that the third biggest carrier in the United States is confident in its outlook after the collapse of a merger with Sprint Corp.

T-Mobile's shares have shed around 10 percent since the collapse of the Sprint merger, which promised estimated benefits of $40 billion. The buyback plan signals management's strong conviction on the business outlook to investors.

Carter, speaking at a Morgan Stanley TMT conference in Barcelona, said the buyback proposal would be put to the board this month. He said Deutsche Telekom, which owns around 64 percent in T-Mobile, would not tender shares and may even buy stock itself.

The issue of control was one of several deal-breakers in the T-Mobile-Sprint talks. By participating in a buyback Deutsche Telekom would concentrate its T-Mobile holding, strengthening its hand in any future merger talks.

Carter said he was "excited about the potential in a rational way to start returning cash to shareholders", citing T-Mobile's strong free cash flow and manageable debt levels.

T-Mobile had briefed credit ratings agencies on the buyback, he said. Moody's last week upgraded its rating on T-Mobile to Baa2 to reflect the company's strong performance and improved financial leverage. Carter said he expected S&P to follow suit.

The shares would be held in treasury and deployed as acquisition currency for future M&A, Carter also said, highlighting interest in targets in the so-called Internet of Things or regional players.

T-Mobile expressed confidence in its ability to grow as a standalone company, having invested $8 billion in 600 MHz spectrum that will position it to launch countrywide fifth-generation coverage by the turn of the decade.

"We are committed to roll out 5G across the nation by 2020," Chief Technology Officer Neville Ray also told the conference.